In order to provide graceful degradation of function in case of total blackouts, I would like to have my smaller UPS plugged into the larger one so my main computer lasts the longest.
Question: Is it safe to plug a UPS into another UPS like this?
In order to provide graceful degradation of function in case of total blackouts, I would like to have my smaller UPS plugged into the larger one so my main computer lasts the longest.
Question: Is it safe to plug a UPS into another UPS like this?
I'm assuming that you're talking about typical, cheap UPSes. These devices only convert DC to AC when they have no input AC or their input AC is out of tolerance.
First, the problem you won't have: You will never have double conversion. If one UPS is on, the other will not be. The UPS connected to the wall, if converting, will keep the second UPS from converting since it will have AC input. The UPS connected to the other UPS, if converting, must not have an AC input and hence the other UPS is not converting.
Since you will never have double conversion, there will be no degradation. Each UPS will produce the output it produces, regardless of what the other UPS is doing.
With the power on, both UPSes will charge from the AC that passes straight through them. The load will run on AC as well.
When the AC fails, the first UPS will start converting. The second UPS will see AC and its battery is already charged. So it will just pass the first UPSes output AC through itself to the load.
When the first UPS dies, the second UPS will stop getting AC. It will run normally, converting DC to AC just as it would if the first UPS wasn't there.
Really there are only two problems you'll have.
First, the UPS plugged into the wall might need to supply power to charge the second UPS's battery some of the time. This will almost never happen, but you do need to make sure the first UPS can handle it. If it shuts down under that load, you'll be in trouble.
Second, the second UPS might not be happy with the output waveform of the first UPS. Some UPSes allow you to adjust their tolerance for crappy input waveforms. If the first UPS is at least a stepped square wave, you'll probably be okay. But you may find that the second UPS refuses to see the output from the first UPS as acceptable. (This only rarely actually happens, but it's hard to guarantee that it won't.)
Can you? Sure. Should you? No. Quality of the voltage will drop, and the back and forth Ac to dc to Ac to dc to Ac to dc will be inefficient. Get a laptop or a bigger UPS.
I have been researching this today because I was considering this. The short of it, don't do it. UPS into a UPS might work, but it can also cause damage to both UPSs or even a fire.
There are several reasons. The first has to do with how the UPS outputs power. When a UPS goes on battery, it doesn't output the same kind of power as what passes through. The second UPS will usually either also go on battery at the same time, or interpret the bad power as a spike and try to ground it. That means the 2nd UPS will send power down it's ground to the first UPS. That could very bad.
There are a number of other reasons, but it isn't something you should do. I am a chemical engineer by education, but I have been doing electrical engineering work in my role at an industrial plant and did the research today and stumbled across this thread so thought I would contribute.
Simpler overall answer regarding UPS or surge protectors or plain old power strips : Don't plug them into each other. There's a risk from fire, data loss, and equipment damage from someone in the future overloading either device, the outlet, or the breaker. That's before we get into the power efficiency loss, or damaging downstream units from lack of active power factor correction.
Most UPS systems output a stepped sine wave when on battery power. Almost all UPS equipment interprets a stepped sine wave as bad power. Therefore, as soon as the first UPS goes on battery and outputs a stepped sine wave, the second UPS in the series will read that input as bad power and convert to battery also. With both UPS systems running on battery at the same time, there is no increase in battery capacity.
I don't get the point. If you plug the smaller one into the bigger one it will just drain the bigger one faster and wont come on till the big one peters out.
If you plug the big one into the small one.. same thing but the small one wont last long.
In all that there is an efficiency loss... A typical UPS is only between about 92 - 95% efficient when on line. While the first UPS is still up you are wasting 4-8% of it's capacity just keeping the second UPS alive.
Ultimately, you end up getting less time out of them and risk blowing up one or both UPS units.
Better to be smart about what you really need powered up through the outage. Only plug things into the backup power plugs you really need kept alive.
Also, you should remember the UPS is really there to give you sufficient time to get to your system and shut it down in a controlled fashion. It is not really intended to let you keep running for a long time. That's what backup generators are for.